Measurement method and apparatus of an external digital camera imager assembly

ABSTRACT

A method for determining whether an imager assembly outside of a camera body meets predetermined focus specifications, wherein the imager assembly includes an image sensor and a camera mounting plate having reference features adapted to cooperate with alignment features in the camera body to locate the image sensor at a predetermined focal plane, including the steps of: mounting the imager assembly onto an imager mounting apparatus having equivalent alignment features; and utilizing low-coherence light interferometry to determine whether the image sensor will meet predetermined focus specifications when mounted in a camera body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 09/697,808, filed Oct. 27,2000 (allowed, Sep. 10, 2002).

FIELD OF THE INVENTION

The present invention relates generally to optical interferometry. Moreparticularly, the present invention relates to a method and apparatusfor determining the location of an imager plane with respect to acamera-mounting plane of an imager assembly.

BACKGROUND OF THE INVENTION

In a conventional digital camera, an image beam is directed through alens and onto an imager or image sensor comprised of an array of sensingelements, for example a Charge Coupled Device (CCD). In order to providea focused image, the lens and the imager need to be properly positioned,relative to each other, within the digital camera.

The steps in a CCD based imager manufacturing process are as follows.Multiple CCD imager arrays are processed together on a single siliconwafer. Imager dies, composed of a single CCD imager array, are dicedfrom the wafer and positioned and glued into specially designedpackages. A flat transparent plate called the imager cover glass is thenglued into the specially designed package at a location that is offsetfrom the imager die to hermetically seal the specially designed package.This hermetically sealed package containing the imager die is thenmounted into a camera-mounting plate that includes a reference plane tofacilitate proper mounting into the camera. The camera itself willinclude a camera reference plane to receive the camera mounting platefrom the imager package. In a film camera, film rails usually define thecamera reference plane. Optionally, the package can include thecamera-mounting plate and reference plane, which would eliminate thislast step of mounting the hermetically sealed package into a cameramounting plate.

In order to ensure that the CCD is positioned properly in the camera,the location of the CCD needs to be determined. Such a location can bedetermined relative to a reference surface or reference plane.

A Coordinate Measuring Machine (CMM) is an example of an apparatusemployed to determine the location of an object relative to a referenceplane. Conventionally, the object is retained in a suitable holder on anoptical bench. In one method to determine the location of an object,three points on a reference plane, approximately 120 degrees apart, aremeasured to define the reference plane; the coordinates of the threepoints are tracked in the x, y and z directions. A point on the objectis then measured relative to the reference plane, and the distance fromthe reference plane is calculated. Conventional CMMs have contact probesfor intimately contacting each of the points defining the referenceplane and the object, such as those described in U.S. Pat. No. 5,428,446issued Jun. 27, 1995 to Ziegart et al. entitled Measurement Instrumentwith Interferometer and Method, U.S. Pat. No. 5,446,545 issued Aug. 29,1995 to Taylor entitled Method of and Apparatus for Calibrating MachinesIncluding a Measuring Probe and a Measuring Apparatus, and U.S. Pat. No.4,929,082 issued May 29, 1990 to Webber entitled Laser Linear DistanceMeasurement System and Apparatus. These references includeinterferometers that monitor the displacement of the machine axes. Incontrast, non-contacting methods, such as optical triangulation, aredescribed in U.S. Pat. No. 4,373,804 issued Feb. 15, 1983 to Pryor et alentitled Method and Apparatus for Electro-Optically Determining theDimension, Location and Attitude of Objects, and U.S. Pat. No. 5,510,625issued Apr. 23, 1996 to Pryor et al. entitled Method and Apparatus forElectro Optically Determining the Dimension, Location and Attitude ofObjects.

Another technology known as low-coherence light interferometry has alsobeen used to measure physical properties of an object. U.S. Pat. No.5,659,392 issued Aug. 19, 1997 to Marcus et al. entitled Associated DualInterferometric Measurement Apparatus for Determining a PhysicalProperty of an Object, and U.S. Pat. No. 5,596,409 issued Jan. 21, 1997to Marcus et al. entitled Associated Dual Interferometric MeasurementMethod for Determining a Physical Property of an Object, disclose anassociated dual interferometric apparatus and method for measuringphysical properties of an object, such as thickness, group index ofrefraction, and distance to a surface. U.S. Pat. No. 5,757,485 issuedMay 26, 1998 to Marcus et al. entitled Digital Camera Image SensorPositioning Method Including a Non-Coherent Interferometer, and U.S.Pat. No. 5,757,486 issued May 26, 1998 to Marcus et al. entitled DigitalCamera Image Sensor Positioning Apparatus Including a Non-Coherent LightInterferometer, disclose a digital camera image sensor positioningapparatus and method which includes a low-coherence lightinterferometer. The apparatus and method include a removable opticalprobe assembly mounted to a digital camera. The low-coherence lightinterferometer is in communication with the optical probe assembly todetermine a depth of an image sensor residing within a digital camera,relative to a reference surface. U.S. Pat. No. 6,075,601 issued Jun. 13,2000 to Marcus et al. entitled Optical Probe Calibration Apparatus andMethod describes an optical probe calibration apparatus used forcalibrating the optical probes used in U.S. Pat. Nos. 5,757,485 and5,757,486 referenced above. These three aforementioned U.S. patentsrequired that the optical probe be mounted in the camera body in orderto determine the location of the imager sensor with respect to thecamera reference surface.

Heretofore, a skilled operator was required to install the imager in thecamera and subsequently assemble the camera before finding out if theimager was properly focused. Several steps were required, includingsecuring the imager with 3 or 4 screws onto the camera-mounting plane,and inserting a measurement optical probe into the camera body andlocking the probe into the lens flange-mounting ring before ameasurement could be initiated. Before mounting the measurement opticalprobe into the camera body, the camera electronics needed to be turnedon and the electronic shutter needed to be opened. Full camera assemblyand substantial skilled operator intervention were required before anassessment of imager focus could be made. If the imager was out offocus, the camera had to be disassembled and the imager replaced. Inorder to calibrate the measurement optical probe, an externalcalibration fixture was also required. The distance from thecamera-mounting ring to the reference surface in the externalcalibration fixture is better suited for measurement with an externaltechnique, such as provided by a CMM machine.

While internal apparatus and methods may have achieved a certain levelof success, the internal apparatus is not readily transportable norsimple to use. Further, the methods are time consuming and quite oftenare dependent on the skill of the operator.

Accordingly, a need continues to exist for an apparatus and method fordetermining the position of an image sensor in a digital camera.Furthermore, there is a need to properly predict the position of animage sensor before permanently physically mounting the image sensorinside the digital camera. The apparatus needs to be robust,transportable and simple to use. The method must be fast, provideobjective results independent of the operator, and provide accurate andconsistent results.

SUMMARY OF THE INVENTION

The need is met according to the present invention by providing a methodfor determining whether an imager assembly outside of a camera bodymeets predetermined focus specifications, wherein the imager assemblyincludes an image sensor and a camera mounting plate having referencefeatures adapted to cooperate with alignment features in the camera bodyto locate the image sensor at a predetermined focal plane, including thesteps of: mounting the imager assembly onto an imager mounting apparatushaving equivalent alignment features, and utilizing low-coherence lightinterferometry to determine whether the image sensor will meetpredetermined focus specifications when mounted in a camera body.

The present invention also provides an imager mounting apparatus toreceive an imager assembly in a predetermined orientation fordetermining whether an imager assembly outside of a camera body meetspredetermined focus specifications, including: an optical probe with apellicle reference surface; a camera body mounting equivalent withequivalent alignment features for receiving and aligning the imagerassembly in a predefined orientation; and a plurality of clamps to lockin the predetermined orientation.

The present invention also provides an interferometric-based measurementsystem for determining whether an imager assembly outside of a camerabody meets predetermined focus specifications, including: a lowcoherence light interferometer; an imager mounting apparatus includingan optical probe having an optical probe chuck; an optical fiber cablefor coupling light from the interferometer to the optical probe chuck;and a computer for processing data collected by the interferometer,wherein the data is used to determine whether the imager assembly meetspredetermined focus specifications.

The present invention also provides a method for calibrating an absolutedistance to a reference surface for determining the position of animager plane relative to an image sensor camera-mounting reference planein an imager assembly, including the steps of: mounting a flat referenceplate onto an imager mounting reference surface; and utilizing lowcoherence light interferometry to determine the distance between theimager mounting reference surface and a pellicle reference surface(known as PP′) of the imager mounting apparatus.

The present invention also provides a method for determining a positionof an imager plane relative to an image sensor camera-mounting plane inan imager assembly, including the steps of: temporarily mounting theimager assembly onto an imager mounting apparatus having an imagermounting reference surface such that the imager sensor camera-mountingreference plane and the imager mounting reference surface are inintimate contact; wherein the imager mounting apparatus includes anoptical probe with a pellicle reference surface in a predeterminedorientation with respect to the imager mounting reference surface suchthat the pellicle reference surface is disposed at a first depthrelative to the imager mounting reference surface; utilizinglow-coherence light interferometry to determine: (i) a second depth fromthe pellicle reference surface to a front surface of the opticallytransparent plate, (ii) an optical thickness of the imager cover glass,and (iii) a third depth from a back surface of the imager cover glass tothe imager plane; and calculating the optical position of the imagerplane relative to the imager sensor camera-mounting reference plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a prior art schematic back view of a camera body includinga camera mounting reference surface;

FIG. 1B shows a prior art schematic side view of a camera body with acamera mounting reference surface and a lens flange mounting ring forattaching a lens to the camera body;

FIG. 2A shows a prior art plan view of an example camera mounting plate;

FIG. 2B shows a prior art cross-section view of an imager assemblyincluding an image sensor and parts of a camera body to indicate theorientation of imager mounting into a camera body;

FIG. 3 shows a schematic of an example measurement apparatus;

FIG. 4A shows a schematic of an example clamp table assembly;

FIG. 4B shows a cross-section view of an example clamp table assembly inrelation to the imager assembly;

FIGS. 5A and 5B show relational measurement geometry and thecorresponding parameters which are measured utilizing one embodiment ofthe present invention; and

FIG. 6 shows an example of raw interferometer measurement data obtainedwhen measuring an imager assembly mounted into the imager mountingapparatus.

DETAILED DESCRIPTION OF THE INVENTION

A stationary probe apparatus, referred herein as an imager mountingapparatus, has been developed which includes an optical probepermanently mounted at a constant distance from a reference planedesigned to mimic the function of the imager mounting plane in a digitalcamera. The optical probe has a pellicle reference plane built in to itwhich is used as a reference surface to calculate distances. Preferably,the pellicle reference plane is the surface of a thick, glass, opticalflat that faces the imager assembly in the probe mount. This allows oneto assess the imager focus location with respect to an ideal focus,without the need of inserting the optical probe into a camera body. Thestationary fixture, with the permanently mounted optical probe, alsosecures the imager mounting plate in place with a simple clamping means,thus eliminating the need to use screws which add to assembly time. Inorder to calibrate the apparatus an optically flat plate is installedinto the same apparatus to determine the distance from the optical probepellicle reference plane to the plane in the same apparatus that theimager mounting plate is clamped to. Thus, no external apparatus isneeded to calibrate the optical probe.

FIG. 1A shows a prior art schematic back view of a camera body 30 with acamera mounting reference structure 35 with a camera reference plane 32for mounting an imager assembly (not shown). In a film camera the camerareference structure 35 ordinarily includes a pair of camera film rails.Also shown in FIG. 1A are the camera threaded holes 36 and the cameraalignment pin receiver holes 34 in the camera reference structure 35 formounting the imager assembly. Preferably, one of the camera alignmentpin receiver holes 34 is slotted in order to facilitate mounting of animager assembly to the camera body.

FIG. 1B shows a prior art schematic side view of a camera body 30 with acamera mounting reference structure 35 with camera reference plane 32and a lens flange mounting ring 31 for mounting camera lenses to thecamera body. The distance from the lens flange mounting ring 31 to thecamera reference plane 32 is defined as LR.

In order to properly focus an imager when mounted into a camera body itmust be located at a defined distance from the lens flange mounting ringwithin a design tolerance. In the manufacture of precision cameras suchas SLR cameras the distance LR is tightly controlled so that properfocus can be assessed by determining the distance from the camerareference plane 32 to the position of an imager die in an imagerassembly.

FIG. 2A shows a plan view of a prior art camera mounting plate 20disclosing the reference features adapted to cooperate with alignmentfeatures in the camera 30 of FIGS. 1A and 1B, and that enablespositioning an image sensor 12 (shown in FIG. 2B) at a predeterminedfocal plane once mounted inside the camera 30. Usually the predeterminedfocal plane is measured with respect to the lens flange-mounting ring 31(shown in FIG. 1B) of the camera body 30. FIG. 2A shows a cameramounting plate 20, mounting holes 26, alignment pins 24 and an imagesensor cutout 28.

FIG. 2B shows a prior art cross-section view of an imager assembly 10including an image sensor 12 and parts of a camera body 30 to indicatethe orientation of imager mounting into a camera body. The cross-sectionview shown in FIG. 2B is that shown by the dashed line in FIG. 2A andlabeled 2B. An imager assembly 10, shown schematically in FIG. 2B,includes an image sensor 12 mounted to a camera-mounting plate 20 whichincludes reference features adapted to cooperate with alignment featuresin the camera 30 which facilitate locating the image sensor 12 at apredetermined focal plane. These reference features include an imagesensor camera-mounting reference plane 22, alignment pins 24 and aplurality of mounting holes 26 to facilitate proper mounting into thecamera 30. During assembly the image sensor 12 is attached to the cameramounting plate 20 by bonding means 21. The lens flange-mounting ring 31is also shown in FIG. 2B to show orientation of the camera body.

Also shown in FIG. 2B, is the camera reference plane 32 that is alignedwith the image sensor camera mounting plate reference plane 22 duringthe imager assembly's 10 installation into the camera 30. Alignment pins24 are installed in camera alignment pin receiver holes 34 which causemounting holes 26 to automatically align with camera threaded holes 36.The imager assembly 10 is secured to the camera 30 with screws (notshown) placed in the camera mounting plate 20 which pass throughmounting holes 26 and are threaded into the camera threaded holes 36.

The image sensor 12 includes an imager die 13 with an imager plane 14,offset from an optically transparent imager cover glass 16 with frontsurface 25 and back surface 11 defining an imager gap 18 between theimager plane 14 and the back surface of the imager cover glass 16. Theimager cover glass 16 can be mounted in an imager package 19 with ahermetic seal at the cover glass bond perimeter 23 around the imager die13. The image sensor 12 also includes imager electrical connections 17on the bottom edge of the imager package 19. The imager die 13 is gluedto the imager package 19 at the imager die 13 to package bond locations15.

During the assembly process the image sensor 12 is bonded to the cameramounting plate 20 in a predetermined orientation using bonding means 21.This bonding is preferably performed with epoxy. The imager focusposition can then be preferably tested with the method and apparatus ofthis invention before curing the epoxy. If the imager position meetspredetermined specifications the epoxy will then be cured. If theposition does not meet predetermined specifications, the position willbe adjusted before curing the epoxy.

FIG. 3 shows a schematic of the measurement apparatus 41 including anoptical interferometer 80, a computer 90, with A-D converters and dataacquisition and control capability for passing interferometer controlparameters and collecting interferometric data through data transmissioncables 66 from the interferometer 80 to the data acquisition boards incomputer 90, an optical multiplexer 60 and an imager mounting apparatus40. The imager mounting apparatus 40 includes a primary base 42 whichpreferably sits on any table, main vertical standoffs 44 which fasten tothe primary base 42 and base adapter 46, an optical probe 48 mounted tothe base adapter 46; a camera body mounting equivalent 50 havingequivalent alignment features to a camera body including an imagermounting reference surface 52 attached to the base adapter 46, withalignment holes 54 to receive alignment pins 24 from the imager mountingplate 20. For a film camera the camera body mounting equivalent 50 isdesigned to mimic the film rails in the camera. The optical probe 48also includes individual optical probe chucks 53 and an optical probepellicle reference plane 55. A plurality of holes 56 are also includedin the camera body mounting equivalent 50 to match the locations of thethreaded screw holes 36 in a camera to align with alignment holes 26 inthe imager mounting plate 20 of imager assembly 10. FIG. 3 also shows aplurality of toggle clamps 51 and clamp bases 57 that are attached tothe base adapter 46 and are used to secure the imager assembly 10. Theimager assembly 10 is mounted in the imager mounting apparatus 40 inplace during the measurement.

During a measurement, light from a low-coherence source (not shown)inside the interferometer 80 is sent to the optical multiplexer 60 byinterferometer single mode fiber cable 64. The optical multiplexer isused to switch between different measurement locations on the imagersurface. This is done by switching the optical connection inside themultiplexer 60 between the various single mode optical fibers 62attached to the back of optical multiplexer 60 which are coupled to theindividual optical probe chucks 53 of optical probe 48 which define theindividual measurement locations on the surface of the imager 12. Duringa measurement sequence each of the optical probe chuck locations 53 aremeasured and analyzed in a defined sequence.

The optical probe 48 is defined as having a pellicle reference plane 55.The preferred pellicle reference plane 55 is the surface of a thick,glass, optical flat that faces the imager assembly 10 when mounted inthe imager mounting apparatus 40.

Referring to FIG. 4A, in one embodiment, a removable clamp tableassembly 77 is preferably used to secure the imager assembly 10 to theimager mounting apparatus 40 utilizing the plurality of toggle clamps 51(see FIG. 3). The removable clamp table assembly 77 includes a table top71, a plurality of clamp table standoffs 73 each with its own standoffalignment pin 75. Standoff alignment pins 75 are located at points tocorrespond with the imager mounting holes 54 in the camera mountingplate 20 of the imager assembly 10 used for mounting in the camera 30.

FIG. 4B shows a cross-section view of the imager assembly 10 geometrywhen mounted for measurement purposes in the imager mounting apparatus40. The cross-section view is indicated by the dashed line in FIG. 4Aand labeled 4B. The optical probe 48 is on the bottom facing up lookingthrough the base adapter 46, and the imager assembly 10 is placed facedown in the camera body mounting equivalent 50 (shown in FIG. 3), andpositioned on the top surface of the base adapter 46, so that alignmentpins 24 of the imager assembly 10 fit into alignment holes 54 of thecamera body mounting equivalent 50, mounting holes 26 in the imagerassembly 10 are aligned with the holes 56 of the camera body mountingequivalent 50. When the alignment is complete the active area of theimage sensor 12 faces the optical probe 48. The clamp table 77 is thenpositioned on top of the imager assembly 10 so that the standoffalignment pins 75 fit in the mounting holes 26 of the imager assembly10. The toggle clamps 51 are subsequently toggled to their contactposition so that the clamp load is distributed over the imager assembly10 at the positions of the mounting holes 26. A clamping force isapplied which mimics the loading that the imager assembly 10 would havewhen screws are inserted into the mounting holes and threaded into acamera body 30.

FIGS. 5A and 5B show a schematic of the measurement geometry and theparameters measured with an interferometric based measurement system.FIG. 5A shows the measurement of the image sensor 12 while FIG. 5B showsthe measurement of a reference plate used to calibrate the measurementsystem. Shown in FIGS. 5A and 5B are the optical probe 48, an opticalprobe chuck 53, the pellicle reference surface 55, and the imagermounting reference plane 52. FIG. 5A also shows the locations of therelevant imager assembly 10 components, including the imager die 13 withimager plane 14 and the imager cover glass 16, with front surface 25 andback surface 11 and the image sensor camera mounting reference plane 22.When the imager assembly 10 is mounted into the imager mountingapparatus 40 the imager mounting reference plane 52 and the image sensorcamera mounting reference plane 22 are coincident in space. The distancePG is defined as the distance from the pellicle reference plane 55 tothe front surface 25 of the imager cover glass 16. The distance ‘g’ isdefined as the distance between the imager plane 14 and the back surface11 of the imager cover glass 16. The thickness of the imager cover glassis defined as ‘t’. During a measurement, the optical thickness of theimager cover glass (nt) is measured with the interferometer 80, where‘n’ is the group index of refraction of the imager cover glass 16.

In order to locate the height of the imager plane with respect to theimage sensor camera mounting reference plane 22, the distance from thepellicle reference plane 55 to the imager mounting reference plane 52 inthe imager mounting apparatus is measured, since these two planes arecoincident during the measurement. The measurement is performed bymounting a flat reference plate 72 with flat reference plane 74 as shownin FIG. 5B onto the imager mounting reference plane 52. The flatreference plane 74 is coincident with the imager mounting referenceplane 52 and the image sensor camera mounting reference plane 22. Thedistance between the optical probe pellicle reference plane 55 and theflat reference plane 74 is defined as PP′ which is equivalent to thedistance between the pellicle reference plane 55 and the imager mountingreference plane 52. Thus a measurement performed using the flatreference plate 72 is used as a calibration to determine the parametersPP′ for each of the optical probe chuck locations.

The objective of the measurement is to determine the position of theimager plane 14 with respect to the image sensor mounting plane 22.Comparing this to specification limits for focus, when mounted inside acamera 30, a determination can be made if the camera 30 will be in focuswhen the imager assembly 10 is mounted inside the camera 30. Inperforming the calculation it is desired to measure the effectiveoptical distance between the imager plane 14 and the imager sensormounting plane 22 which we call the die-to-plate distance (DP). Thepresence of the imager cover glass increases the effective focaldistance of a lens by an amount Δ_(G) given by the relationshipΔ_(G)=t(1−1/n) where t is the thickness of the imager cover glass (16)and n is the group index of refraction of the cover glass at thewavelength of the light source used in the interferometer. The physicaldie to plate distance (DP)_(p) is given b

(DP)_(p) =PG+g+t−PP′.  (1)

In the digital camera application we are interested in the effectiveoptical die to plate distance DP which is equal to

DP=(DP)_(p)+Δ_(G) =PG+g+(nt)/(n)² −PP′  (2)

where PG is the distance from the pellicle reference plane 55 the frontsurface 25 of the imager cover glass 16, ‘g’ is the gap between theimager plane 14 and the back surface 11 of the imager cover glass 16,‘n’ is the group index of refraction of the imager cover glass, ‘t’ isthe thickness of the imager cover glass, ΔG is the focus distanceincrease due to the presence of the imager cover glass 16 and PP′ is thedistance between the pellicle reference plane 55 and the imager mountingreference surface 52 at the probe chuck 53 measurement location. PP′ ismeasured by installing a flat reference plate 72 at the measurementlocation. The flat reference plate preferably includes a plurality ofholes to mate to the alignment pins 75 of the removable clamp tableassembly 71.

Note that no externally measured parameters are required in order todetermine the die to plate spacing. This is a drastic improvementcompared to the internal camera measurements made in the prior art whichrequire an external measurement such as a CMM measurement of a referencecradle to provide a reference distance required for determining imagerfocus error in a camera or film rail locations.

FIG. 6 shows an example of raw interferometer measurement data obtainedwhen measuring an imager assembly mounted into the imager mountingapparatus. During the measurement the interferometer is made to scan adistance large enough to measure the relevant distances g, nt and PG.The data is obtained using an interferometer operating in anautocorrelation geometry. An example of an interferometer operating inan autocorrelation geometry is shown in FIG. 11 of U.S. Pat. No.5,757,486 referenced above. The interferometer continually scans backand forth a distance greater than the largest measured distance PG andis made to cross the zero-crossing point in the interferometer, theposition at which the path lengths of the 2 arms of the Michelsoninterferometer are equal in length. Motor scan reversal points are shownin FIG. 6 as the curved lines. Peaks 110, 120, 210 and 220 are zerocrossings of the interferometer and all measured distances arereferenced to the nearest adjacent zero crossings. The interferometertrace segment shown in FIG. 6 correspond to one complete interferometermotor scan cycle plus passing again across the zero crossing points. Theinterferometer motor reverses scan directions between pairs of peaks 110and 120, 160 and 170 and 210 and 220.

The imager gap ‘g’ is determined by measuring the distance between peaks120 and 130 and/or 200 and 210, the optical thickness nt of the imagercover glass 16 is determined by measuring the distance between peaks 120and 140 and and/or 190 and 210, and the pellicle gap (PG) is determinedby measuring the distance between peaks 120 and 160 and/or 170 and 210.The distance between peaks 120 and 150 and or peaks 180 and 210 definethe distance g+nt. During a measurement usually a set interval of time,such as, 1 second is used to repetitively scan the interferometer backand forth at a typical rate such as 10 Hz. This allows 20 measurementsper second, and an average value of the measurements would be stored ina computer file. Interferometer peak locations are determined by themethods of the references. Suitable peak location calculation proceduresare described in U.S. Pat. Nos. 5,596,409 and 5,659,392 referencedabove.

PARTS LIST

10 imager assembly

11 imager cover glass back surface

12 image sensor

13 imager die

14 imager plane

15 imager die to package bond locations

16 imager cover glass

17 imager electrical connections

18 imager gap

19 imager package

20 camera mounting plate

21 bonding means

22 image sensor camera mounting reference plane

23 cover glass bond perimeter

24 alignment pins

25 imager cover glass front surface

26 mounting holes

28 image sensor cutout

30 camera body

31 lens flange-mounting ring

32 camera reference plane

34 camera alignment pin receiver holes

35 camera mounting reference structure

36 camera threaded holes

40 imager mounting apparatus

41 measurement apparatus

42 primary base

44 main vertical standoff

46 base adapter

48 optical probe

50 camera body mounting equivalent

51 toggle clamps

52 imager mounting reference surface

53 optical probe chucks

54 alignment holes

55 optical probe pellicle reference plane

56 holes

57 clamp base

60 optical multiplexer

62 single mode optical fibers

64 interferometer single mode fiber cable

66 data transmission cables

71 table top

72 flat reference plate

73 clamp table standoff

74 flat reference plane

75 standoff alignment pins

77 clamp table assembly

80 optical interferometer

90 computer

110 peaks

120 peaks

130 peaks

140 peaks

150 peaks

160 peaks

170 peaks

180 peaks

190 peaks

200 peaks

210 peaks

220 peaks

What is claimed is:
 1. A method for determining a position of an imagerplane relative to an image sensor camera-mounting plane in an imagerassembly, comprising the steps of: a) temporarily mounting the imagerassembly onto an imager mounting apparatus having an imager mountingreference surface such that the imager sensor camera-mounting referenceplane and the imager mounting reference surface are in intimate contact;wherein the imager mounting apparatus includes an optical probe with apellicle reference surface in a predetermined orientation with respectto the imager mounting reference surface such that the pelliclereference surface is disposed at a first depth relative to the imagermounting reference surface; b) utilizing low-coherence lightinterferometry to determine (i) a second depth from the pelliclereference surface to a front surface of the optically transparent plate,(ii) an optical thickness of the imager cover glass, and (iii) a thirddepth from a back surface of the imager cover glass to the imager plane;and c) calculating the optical position of the imager plane relative tothe imager sensor camera-mounting reference plane.
 2. The method claimedin claim 1, wherein the step of calculating the optical position of theimager plane relative to the imager sensor camera-mounting referenceplane DP, defined by the relationship DP=PG+g+(nt)/(n)²−PP′, wherein thefirst depth is defined as PP′, wherein the second depth is defined asPG, and wherein the third depth is defined as g, and the opticalthickness of the imager cover glass is defined as (nt), where ‘n’ is anindex of refraction of the imager cover glass.